How about some more information and a description of the situation? I may be able to help you but not based on what you've said so far.

Hi! I My old Samsung TV was connected to a Yamaha reciever (surround sound). When the said TV broke, I disconnected the RCA jack from the TV end. When I bought the new TV (Toshiba 55", 60Hz LED Chromecast) I re-plugged in the RCA jack but there's no sound coming out from the speakers that are connected to the receiver. What happened?

On the TV side of the audio cables, are they plugged into sockets labeled as "Audio Out"?

If yes, then there may be a menu setting that needs to be changed. Press Menu button on remote, then open the Sound section, experiment with different changes (while your stereo is on, of course)

If no, then it's plugged in at the wrong point. If there aren't RCA analog sound output sockets on the TV, you may need to use a different type of connection or wire(s) than what you used with the old TV.

Try calling the store you bought it from, or Toshiba, for help.

The Toshiba model no. would be helpful. Looking at a picture of Toshiba LED TV, it may only have Audio/Video RCA jack in. The audio output may be through the head jack output which would require a headphone jack to RCA plugs or possibly a TOS-link fiber optic cable. My Samsung audio out to stereo is only the TOS-link. If receiver isn't fiber optic capable, you can get a TOS-link to RCA plug adapter, or possibly its time to upgrade the receiver.

Usually a new TV comes with instructions including pages and pages of diagrams on how to hook up your TV to all kinds of devices.